Excessive daytime sleepiness is a disabling symptom associated with various sleep-wake disorders (e.g. sleep apnea syndrome, narcolepsy). In addition, a substantial segment of the population experiences severe sleepiness for various reasons associated with insufficient or inadequate sleep as well as with alcohol and other central nervous system drugs. Irrespective of its cause, the symptom, excessive sleepiness itself, has serious social and medical consequences. However, even though sleep evaluation clinics and treatments have become available, little is understood about the nature of pathological sleepiness. A number of different causes of sleepiness have been identified. A direct objective method which uses sleep laboratory procedures (the Multiple Sleep Latency Test [MSLT]) has gained acceptance within the clinic as well as the research laboratory as the standard method of quantifying sleepiness. A specific MSLT value (i.e., sleep latency < 5 mins) has been identified as a pathological level of sleepiness. This research program will attempt to further understand pathological sleepiness by exploring how sleepiness is reversed as a function of its etiology and chronicity. Using the MSLT as the primary dependent measure studies will be conducted to determine the alerting effects of naps (replacement sleep) and caffeine (symptomatic treatment) on sleepiness of an identical level, but associated with sleep loss, a sedating drug (triazolam), narcolepsy, and sleep apnea. To understand chronicity, a study will be done assessing the effect of caffeine and naps on sleepiness of identical etiology and level, but differing duration. The results of this program will not only provide useful information as to the reversal of sleepiness but also as to the differing mechanisms of sleepiness. Some sleepiness is related to sleep debt while other forms of sleepiness are associated with other mechanisms (e.g. inability to maintain neural state). It is anticipated that the differential response to naps and stimulants will provide leads as to the different mechanisms of sleepiness.